The present invention relates to a new and useful method of manufacturing semiconductor elements from amorphous silicon (a-Si) which convert light into electric energy, and a device for carrying out the method.
Amorphous silicon (a-Si) is a promising new and substantially less expensive material for the mentioned purpose than crystalline silicon. The value of a-Si is primarily in that because of its substantially higher optical absorption, it is needed in only about one micron thick layers, while with crystalline silicon, a thickness of at least 50 to 100 microns is necessary. Amorphous silicon can be deposited directly from a gaseous phase and on relatively inexpensive substrates and, in contradistinction to crystalline silicon, no further costly material processing is needed.
It is known, from German OS No. 27 43 141, to employ a glow discharge for producing a-Si semiconductor elements, for example, from silane. For this purpose, a glass bell jar is used, with a connected electrode and an opposite heating plate accommodated therein. One outlet of this jar communicates with a diffusion pump, another with a mechanical pump, and a third outlet is connected to a gas supply system serving as a source of the gases needed for the glow discharge. The substrate, for example, stainless steel, on which the a-Si is precipitated is placed on the heating plate. A source of energy which is usually radio-frequency operated is connected to the electrode and the substrate.
With such semiconductor elements, efficiencies of a few percent have been obtained up to the present time. To make them economically interesting, a substantially improved efficiency of the cells is needed. It is known that among other things, incorporation of hydrogen is of great importance for the electrical properties of a-Si (see also W. Spear et al. Solid State Comm. 17,1193,1975). Hydrogen serves to saturate the so called dangling bonds and reduces the number of traps in the mobility gap. The number of these traps is directly responsible for the range of the field zone extending in the a-Si cell from the pn-junction or the Schottky contact. A small number of traps means that a wide field zone can be obtained. Since in an a-Si cell, only those electric charge carriers contribute to the efficiency of the cell which are produced directly in the space-charge zone, and the diffusion of charge carriers from regions outside the field zone being of secondary importance, a wide space-charge zone, as far as possible ranging over the entire thickness of the a-Si cell, i.e. about one micron, is desirable. Only then may charge carriers owing their existence to the longer waves of sunlight which penetrate deeper into the a-Si, also contribute to the efficiency of the cell. In the best prior art at a-Si cells, widths of about 0.2 microns of the field zone were estimated.